The resting potential of a typical neuron is around -65mV.
Electrical charge is unevenly distributed between the inside and outside of a
neuron, with the inside being more negative under normal resting conditions. The
reason for this difference is the distribution of ions across the membrane.
Calcium and Sodium ions are more concentrated on the outside, while Potassium
ions are more concentrated on the inside, as well as impermeable anionic
proteins. These concentration gradients arise because of ion pumps, mainly the sodium-potassium
pump, which hydrolyzes ATP in the presence of internal sodium. The energy
created exchanges an internal sodium for a potassium ion.

Other
important terms are:Depolarization: when the membrane potential becomes more positive.Hyperpolarization: when the membrane potential becomes more
negative.

At rest, the membrane is highly permeable to potassium,
and allows a small leakage of sodium. Each ion has it's own Equilibrium
Potential, the steady electrical potential that would be produced if the
membrane were permeable to only that ion. We can use the Nernst Equation
(shown to the right) to calculate this value.

However, the membrane is permeable at rest to
several ions, and their relative permeability must be taken into
consideration when calculating the membrane potential. If we consider only
sodium and potassium, the membrane is 40 times more permeable to potassium
than to sodium, which predicts a resting membrane potential of about -65
mV (which is exactly what we observe!). The equation used to calculate the
membrane potential is the Goldman Equation.

Threshold: Threshold is the membrane potential at which enough
voltage-gated sodium channels are open so that the relative permeability
of the membrane is higher for sodium ions than it is for potassium ions.Rising Phase: When the inside of the membrane has a negative
potential, there is a large driving force for on sodium ions. Therefore,
sodium rushes in through the open sodium channels, causing a rapid
depolarization of the membrane.Overshoot: Because of the high permeability to sodium, the membrane
potential goes to a value that is close to the Equilibrium potential for
sodium (~ +55 mV).Falling Phase: First, the voltage-gated sodium channels inactivate.
Second, the voltage-gated potassium channels open (the delayed-rectifier
potassium channels). The driving force pushes potassium out of the cell,
causing the membrane potential to become negative again.Undershoot: The open potassium channels add to the normal resting
membrane permeability to potassium, and drives the membrane potential
close to the equilibrium potential for potassium, thus hyperpolarizing the
membrane.

Refractory periods: The absolute refractory period is due to
the inactivation of sodium channels. These channels cannot be opened again
until the membrane potential is sufficiently negative to deinactivate
them. The relative refractory period is due to the
hyperpolarization from the open potassium channels. This means that more
depolarizing current is necessary to initiate another action potential.

In order for information to be transferred in
the nervous system, the action potential, once generated, must travel down the
axon. The propagation of the action potential occurs because the influx of
positive charge during the rising phase depolarizes the next segment of the
membrane. In this way, the action potential works its way down the axon to the
terminal, and initiates synaptic communication with another neuron or cell. An
action potential travels only in one direction; it cannot turn back on itself
because the membrane behind it is still refractory due to the inactivation of
sodium channels. Action potentials propagate without decrement, and are called
an "All or none" signal.

Axon diameter and pores: The sodium influx
results in the depolarization of the next segment of membrane, and the
speed of action potential propagation depends in part on how far ahead
this depolarization spreads. There are two paths the positive charge can
take: one, down the inside of the axon; the other, across axonal
membranes. Thus, if the axon is wide and there are few open membrane
pores, most of the current will flow inside the axon, and result in faster
conduction speed.
Myelination: Because all neurons cannot be gigantic to improve
conduction speed, there is another mechanism to improve axonal conduction.
Myelin sheaths are provided by glial cells, and facilitate current flow
inside the axon, as opposed to across axon membranes. The myelin sheath is
not continuous; there are breaks where ions can pass through the membrane
to generate action potentials. Voltage-gated sodium channels are
concentrated in these nodes of Ranvier. Conduction along these myelinated
fibers is referred to as saltatory
conduction.